It is known that magnesium based alloys posses a number of advantages that make them of interest when considering for instance surgical implants. Of particular interest of such magnesium based implants is the possibility of using them to act both as a scaffolding structure on which new bone or tissue can grow and as a fixture structure to hold together a bone or a ligament long enough to allow natural healing to take place.
Magnesium and its alloys are of particular interest in this type of applications as they are bio-compatible and as they have a modulus of elasticity closer to bone than currently used materials. Another major advantage of using magnesium and its alloys as implant materials, for instance for the fabrication of surgical implants, are their ability to bio-degrade in situ. This in turn means that the implant does not remain in the body. A further surgery to remove the implant is not required.
However, recent animal implantation studies seem to exhibit sometimes only a partial direct bone contact of a magnesium based alloy after a certain time if implantation. A fibrous tissue layer separates the newly grown bone from the implant. Additionally, hydrogen gas formation and sometimes even gas bubbles seem to be present on the surface of the implant and in the surrounding tissue after 6 to 12 weeks of implantation. Hydrogen gas evolution or release occurs during the bio-degradation process. The volume of evolved or released hydrogen gas is related to the dissolution of the magnesium. Without being restricted to a theory it is believed that all of these problems are mainly or essentially due to a too fast initial degradation process of the magnesium implant in-vivo. The degradation rate of the magnesium based alloys seems to be too fast, in particular at the beginning directly after implantation. More hydrogen gas is generated than can be readily resorbed or absorbed by the surrounding tissue. This results in the formation of gas bubbles or gas pockets, for instance subcutaneous gas bubbles and/or gas bubbles in the soft-tissue, which could damage the surrounding tissue. This is the major drawback of magnesium and actually hampers the broad application of magnesium based implants.
A recent approach bases on a magnesium based alloy having an adapted composition and morphology. One specific alloy is designed. The composition and the morphology are adapted or designed such that hydrogen gas evolution is avoided (see for instance N. Hort et al., Acta Biomaterialia, Volume 6, Issue 5, Pages 1714-1725, May 2010: “Magnesium Alloys as Implant Materials—Principles of Property Design for Mg—RE Alloys”).
On one hand the design of such an alloy is time-consuming and therefore expensive. On the other hand such a specific alloy possesses only one specific degradation rate. However, in general the degradation rate is dependent on the place of implantation in the body or on the purpose of the implant. For instance, the degradation times of an implant acting as a fixture to hold together a bone long enough to allow natural healing to take place and of an implant embodied as a screw to fix a ligament to a cartilage can be different.
Accordingly, it is an object of the present invention to provide a preferably bio-degradable implant of advanced properties, for instance of enhanced bio-compatibility and/or for providing improved implant-tissue-contact.
A bio-degradable implant should have a reduced degradation rate compared to untreated implants. Particularly, the degradation rate, in particular the initial degradation rate, should be reduced such that a gas accumulation in the tissue is at least reduced or avoided.
In particular it should be possible to control or to adapt the degradation rate or the bio-degradability of such an implant.
Preferably the ingrowth of human tissue and/or bone should be promoted by such an implant.
The fabrication of such an implant should be based on an easy and cost reduced concept.